The present invention relates to a layer 2 loop detection system that enables layer 2 loop detection by an Simple Network Management Protocol (SNMP) manager that monitoring target is a layer 2 network having multiple layer 2 switches.
In a layer 2 network for constructing an enterprise network or the like, as one technique of securing redundancy of the network through loop (layer 2 loop) elimination, the Spanning Tree Protocol (STP) standardized by Institute of Electrical and Electronics Engineers (IEEE) 802.1D is caused to run.
This is because Media Access Control (MAC) frames have no equivalent of the Time to Live (TTL) under the Internet Protocol (IP), so that if a network is looped, this results in a situation where the MAC frames are infinitely circulated at a location at which the network is looped.
Under the STP, by exchanging packets for monitoring so-called “Bridge Protocol Data Unit (BPDU)” between adjacent nodes (layer 2 switches), the location at which the network is looped is detected and a port called “blocking port” that blocks the MAC frames (traffic blocking port) is created, thereby logically blocking the loop.
With the technique utilizing the STP, the occurrence of a layer 2 loop is avoided, although there are cases where the STP is collapsed due to, for instance, a CPU failure of a bridge that is a layer 2 switch. Once the STP is collapsed, the blocking port is lost, so that a loop occurrence becomes unavoidable. As a result, the layer 2 network is placed under a congestion state, which may lead to a failure such as response degradation or meltdown.
When such a failure occurs, bridge log overwriting occurs or communication becomes impossible when the communication is performed using the Simple Network Management Protocol (SNMP) so that there also occurs a situation where it is impossible to identify the location of the failure.
The following are related arts to the present invention.    [Patent Document 1]    Japanese Patent Laid-Open Publication No. 05-316136    [Patent Document 2]    Japanese Patent Laid-Open Publication No. 2002-335258    [Patent Document 3]    Japanese Domestic Laid-Open Publication No. 2001-509657    [Patent Document 4]    Japanese Patent Laid-Open Publication No. 2002-164890    [Non-Patent Document 1]    http://www.nic.ad.jp/ja/materials/iw/2002/proceeding/T18-1.pdf